1. Field
The present invention relates generally to data communication, and more specifically to techniques for determining a non-uniform distribution of data rates to be used for multiple data streams to be transmitted via multiple transmission channels of a multi-channel communication system, e.g., a multiple-input multiple-output (MIMO) system.
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on the other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels may also be referred to as a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
For a full-rank MIMO channel, where NS=NT≦NR, an independent data stream may be transmitted from each of the NT transmit antennas. The transmitted data streams may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs) for a given amount of transmit power. Moreover, if successive interference cancellation processing is used at the receiver to recover the transmitted data streams (described below), then different SNRs may be achieved for the data streams depending on the specific order in which the data streams are recovered. Consequently, different data rates may be supported by different data streams, depending on their achieved SNRs. Since the channel conditions typically vary with time, the data rate supported by each data stream also varies with time.
If the characteristics of the MIMO channel (e.g., the achieved SNRs for the data streams) are known at the transmitter, then the transmitter may be able to determine a particular data rate and coding and modulation scheme for each data stream such that an acceptable level of performance (e.g., one percent packet error rate) may be achieved for the data stream. However, for some MIMO systems, this information is not available at the transmitter. Instead, what may be available is very limited amount of information regarding, for example, the operating SNR for the MIMO channel, which may be defined as the expected SNR for all data streams at the receiver. In this case, the transmitter would need to determine the proper data rate and coding and modulation scheme for each data stream based on this limited information.
There is therefore a need in the art for techniques to determine a set of data rates for multiple data streams to achieve high performance when limited information is available at the transmitter for the MIMO channel.